It is estimated that nearly one billion birds die annually in North America after colliding with windows in the exterior walls of buildings. In 2014, a team of more than 60 researchers led by Assistant Professor of Biology Bradley Cosentino and Professor of Biology Stephen Hager from Augustana College in Illinois, and including Lauren Walter ’16, organized a study through the Ecological Research as Education Network to explore the features that were most likely to result in avian deaths. Data from 40 college and university campuses throughout North America were collected and summarized for an article that appeared in the August 2017 issue of the journal Biological Conservation.
Consistent with previous studies, Cosentino’s group affirmed that building size was a major factor in the number of bird deaths. The study also indicated that mortality was higher in areas of low urbanization that included elements such as landscaped and natural grass, water features and woody vegetation.
“We knew coming into this study that bigger buildings kill more birds,” says Cosentino. “That pattern is almost universally found among studies on bird-window collisions (BWCs). However, our findings show that there’s some nuance to the positive relationship between building size and BWCs. Mortalities increase with building size in both urban and rural areas, but they increase at a faster rate in rural areas. That means a large building of a given size should cause more mortalities in rural than urban areas.”
Cosentino and his group came up with two possible scenarios to explain this. One suggests that since migratory birds are attracted to lights, the collisions in urban areas where there may be many buildings with lights are more spread out or “diluted” than they would be in a rural setting where just one building might attract more birds.
“An alternative hypothesis is that bird behavior differs between urban and rural areas,” Cosentino says. “It could be that urban birds exhibit behaviors that reduce their risk of hitting a window. Urban birds may have learned to avoid windows from previous collisions that didn’t result in mortality, or urban and rural birds may differ in traits that affect the likelihood of collisions (e.g., sight).”
Regardless of the reason for the collisions, the study shines light on a timely topic, says Cosentino. At heart, Cosentino believes, it’s an animal welfare issue that merits serious study. “Some jurisdictions outside the U.S. have moved to make it illegal to have windows that emit reflected light that cause bird-window collisions,” he says. “So there’s a real big push to understand why birds strike windows, where strikes are most likely to occur, and how we can mitigate the problem.”